1. Field of the Invention
The present invention relates to wavelength selective switches, and more specifically, to a wavelength selective switch for performing optical switching.
2. Description of the Related Art
Optical networks based on wavelength division multiplexing (WDM) have been rapidly increasing to serve surging Internet traffic.
Currently, WDM is used mainly in point-to-point networks and will be used in ring networks and mesh networks in the near future. Each node constructing the network will be able to add or drop any wavelength and perform processing such as optical cross connect (OXC), which does not include any optical-to-electronic conversion, and a path will be allocated and deallocated dynamically in accordance with wavelength information.
As a device for implementing these optical networks, a wavelength selective switch (WSS) that allows any wavelength to be switched in any direction has been receiving attention in recent years.
FIGS. 6 and 7 are conceptual drawings of the WSS. FIG. 6 is a top view of a WSS 5, illustrating angular dispersion of optical beams output from a spectroscopic element 51, and FIG. 7 is a side view of the WSS 5, illustrating port switching. The WSS 5 includes the spectroscopic element 51, a collective lens 52, a plurality of mirrors 53 arranged in the direction of angular dispersion of the optical beams output from the spectroscopic element 51, an input port 54, and output ports 55.
Multi-wavelength WDM light input from the input port 54 is separated by the spectroscopic element 51 and gathered by the collective lens 52 onto the mirrors 53 corresponding to different wavelengths. The inclination (angle) of each mirror 53 is changed to output the reflected light from a corresponding output port 55. The mirrors 53 are disposed in the direction in which the beams spread after angular dispersion depending on the wavelengths made by the spectroscopic element 51 and can be turned in a direction differing from the direction in which the beams have spread.
As the spectroscopic element 51, a diffraction grating is generally used. The diffraction grating is an optical element having a number of parallel grooves made at regular spacing on a glass substrate and performs wavelength separation by optical diffraction by giving a plurality of wavelength components input at a certain angle output angles depending on the wavelengths.
The mirrors 53 are generally a group of micro electromechanical system (MEMS) mirrors, and a single mirror is disposed for a single wavelength separated by the spectroscopic element 51. The angle of inclination of the MEMS mirror can be varied, and the inclination angle determines the output port corresponding to each wavelength component.
One measure of the performance of a WSS is pass band. FIG. 8 is a view showing the pass band of the WSS 5. The vertical axis of the graph represents the optical spectral value, and the horizontal axis represents the wavelength. The figure shows the pass band characteristics of the WSS 5 in its initial operation when the beams output from the collective lens 52 fall on desired positions (centers) of the mirrors 53.
The central wavelength of the channel to pass is placed in the central position of the bandwidth of the flat pass band, and the transmittance is optimized (if the pass-band bandwidth is 50 GHz and if the central wavelength of the channel to pass is placed in the central position, the channel has a pass band of 50 GHz).
The pass band of the WSS 5 increases as the width (area) of the mirrors 53 becomes greater than the diameters of the beams output from the collective lens 52 and as the deviation of the central wavelength decreases.
The pass band increases as the width of the mirrors increases, as the diameters of the beams on the mirror decrease, and as the beam of the central wavelength of the channel specified by the ITU-T grid is incident closer on the center of the mirror corresponding to the channel (an ITU-T recommendation on the grid specifies the spacing and wavelengths of multi-wavelength channels in WDM communication).
With a wide pass band (pass-band bandwidth), the upper limit of the bit rate that can be supported can be raised. Light with a high bit rate has a wide spectrum width, and the wide pass band can cover the wide spectrum width.
The wide pass band also allows a number of WSS stages to be connected to be increased because the accumulated amount of bandwidth deviations is small even if the WSSs are connected in multiple stages. With the wide pass band of the WSS, good transmission characteristics can be kept.
A conventional optical switching technique proposed in Japanese Unexamined Patent Application Publication No. 2005-283932 switches the channel for each single-wavelength light component included in WDM light and reduces the number of components by sharing a wavelength demultiplexing element provided in the spectroscopic system.
A requirement for attaining sufficient pass band characteristics with the WSS is that the beam of a wavelength corresponding to the ITU-T grid agrees with the center of the mirror corresponding to each ITU-T grid wavelength.
The components forming the WSS, however, have temperature characteristics. Even if initialization is performed to gather the beams onto the centers of the mirrors, a change in temperature in the operating environment or the like may cause the beams to deviate from the centers of the mirrors, decreasing the pass band.
FIG. 9 is a view showing that the light gathering positions of beams deviate when the temperature changes. The change in temperature changes the angles of beams output from the spectroscopic element 51. The figure shows that the light gathering positions have deviated downward from the centers of the mirrors 53.
A pass band characteristics graph shows that the central wavelength of the beam deviates to the right by a frequency of 10 GHz, for instance. This decreases the pass band of the channel from the initial value of 50 GHz to 40 GHz.
Factors for the change in pass band accompanying a change in temperature include (1) a change in angle or position of each beam depending on the linear expansion coefficients of fixture members which fix the diffraction grating, the collective lens, and the mirror and (2) a change in angle or position of each beam depending on the temperature characteristics (linear expansion coefficient, refractive index temperature coefficient) of the diffraction grating itself).
The factor (1) can be suppressed relatively easily to such a level that the effect on the pass band characteristics becomes small, by using a fixture member made of a material having a low linear expansion coefficient such as Invar.
FIG. 10 is a view showing a deviation in light gathering positions of beams. The shown deviation in light gathering positions of beams is caused by expansion of a fixture member 57 for fixing the collective lens 52. If the fixture member 57 is made of a material having a low linear expansion coefficient such as Invar, if the focal distance of the collective lens 52 is 200 mm, and if the beam output angle of the diffraction grating is 5° both at the shortest wavelength and at the longest wavelength, the deviation ΔX in light gathering positions of beams caused by expansion of the fixture member 57, if the fixture member 57 expands with heat, would be about 0.01 μm.
FIG. 11 is a view showing another deviation in light gathering positions of beams. The shown deviation in light gathering positions of beams is caused by expansion of a fixture member 56 which fixes the mirrors 53. If the fixture member 56 is made of a material having a low linear expansion coefficient such as Invar, if the center-to-center distance of the mirrors 53 is 10 mm, if the change ΔT in temperature is 80° C., the deviation ΔX in light gathering positions of beams caused by expansion of the fixture member 56 would be about 0.8 μm, because the linear expansion coefficient of Invar is about 1×10−6/° C.
A 5-μm deviation in light gathering positions of beams usually corresponds to a 1-GHz change in frequency, and the change of 1 GHz may be considered as degradation of the pass band characteristics. A deviation of up to 1 μm in light gathering positions of beams caused by expansion of the fixture members would not give a great effect on the pass band characteristics.
The factor (2) means that, even if expansion of a fixture member which fixes the diffraction grating does not have a great effect, the spacing between the grooves made in the diffraction grating itself changes by a change in temperature. This will change the angles of the output beams, giving a great influence on degradation of the pass band characteristics.
Even when the diffraction grating is made of quartz, which has a low linear expansion coefficient, the deviation in light gathering positions of beams is about 20 μm for a diffraction grating pitch of 1/1000 mm. If the diffraction grating pitch per channel is set to 500 μm, a 30-μm deviation in light gathering positions of beams corresponds to a 6-GHz decrease in band.
The temperature characteristics of the diffraction grating used as the spectroscopic element 51 have a greater influence on the deviation in light gathering positions of beams than other factors. Because the WSS may use a plurality of diffraction gratings or may use another element that has temperature characteristics, the total change in beam positions would be greater than the values indicated above.
Conventionally, the deviation in light gathering positions of beams caused by the temperature characteristics of a diffraction grating has been suppressed by selecting a diffraction grating made of a material having small temperature characteristics or by performing constant temperature control by means of a heater, a Peltier element, or the like to keep a constant temperature state.
The value of the diffraction grating pitch indicated above is obtained when quarts, which has a low linear expansion coefficient among general glass materials, is used. It is difficult to use a glass material having a lower linear expansion coefficient than quartz, because of the cost and optical characteristics such as transmittance. If a heater, a Peltier element, or the like is used to perform constant temperature control, the control system added for that purpose increases the power consumption.